1. Field of the Invention
The present invention relates to a material for alternatively permitting and preventing transmission of light and other electromagnetic radiation in a controlled manner for very short time periods. More specifically, the invention relates to the use of vanadium oxide deposited on a substrate, for selectively permitting and blocking transmission of infrared (IR), microwave and radio frequency (RF) radiation therethrough.
2. Description of the Prior Art
Certain oxides of vanadium, in its various valences, are known to have varying physical properties at selected temperatures. These characteristics include the transmission of light and resistivity. This is because of a change in the crystalline structure at a certain specific temperature, which is unique to each vanadium compound. VO.sub.2, which is the primary subject of this specification, has a transition temperature at approximately 68.degree. C. At temperatures below this threshold, the compound exhibits dielectric properties and is transparent to a wide range of electromagnetic radiation, including IR and microwave frequencies. At temperatures above the 68.degree. C. threshold, the compound is metallic in character, exhibiting electrical conducting properties and being opaque to most electromagnetic radiation. If properly formulated, this transition from the dielectric to the metallic state occurs over a very narrow temperature range.
VO.sub.2 is the preferred compound for the application disclosed herein because of this narrow temperature range in which the transition takes place. Other vanadium-based compounds, such as V.sub.2 O.sub.3, exhibit similar properties, but at low transition temperatures which are not as easily controlled outside the laboratory.
The switchable properties of this and similar compounds have been utilized in a number of different applications. Nalepa, et al., U.S. Pat. No. 3,993,603, issued Nov. 23, 1976; McHugh, et al., U.S. Pat. No. 4,056,378, issued Nov. 1, 1977 and Emtage, et al., U.S. Pat. No. 3,975,657, issued Aug. 17, 1976, all disclose the use of a VO.sub.2 and MoO.sub.2 mixture to reduce current inrush in incandescent light bulbs. Light bulbs having tungsten filaments are susceptible to damage because of melting from large current overshoots when the bulb is activated. The vanadium/molybdenum compound exhibits higher resistance at lower temperatures, such that when placed in series with the filament, the resistor reduces the initial current peak. As the bulb heats, the resistance is lowered and the bulb achieves its maximum brightness. The molybdenum is added to make the transition from high resistance to low resistance a more gradual one.
Ueoka, et al., U.S. Pat. No. 4,022,716, issued May 10, 1977 utilizes vanadium oxide as an additive in the manufacture of semiconducting ceramics. The use of vanadium oxide and other materials such as iron and manganese in such compounds broadens the range of resistivity in the base materials. The use of vanadium oxide specifically in these compounds gives a broader range than the other elements, and also allows for finer gradations between selected resistivities. This is because of the small changes in resistivities occurring from relatively large additions of the vanadium oxide.
What has not been recognized by the art, however, is the application of the switchable vanadium oxide compounds for selectively permitting and preventing the passage of electromagnetic radiation therethrough, based on the metallic and dielectric properties exhibited by the compound. This is especially applicable in the use and detection of IR, microwave or RF transmitters and receivers.